At this point the "accountlist" on vpnclient and "sessionlist" on vpnserver both show me the active connection "Status: Connected".
Then I've manually set the IP address on both ends:
vpnserver: ifconfig tap_test1 192.168.10.1/24
vpnclient: ifconfig vpn_nic1 192.168.10.10/24

Now I can ping 192.168.10.10 from 192.168.10.1 no problem, but not the other way around. 192.168.10.1 can't even ping itself from a local console. Otherwise everything works fine: 192.168.10.10 can connect to 192.168.10.1 via ssh and other ports.

dmesg is empty. Either something went wrong on a rather low level (bridging maybe?) or I need to alter SoftEther settings.